Transport refrigeration systems condition the air of a refrigerated space of a vehicle, hereinafter called the "conditioned space", such as a straight truck, the trailer of a tractor-trailer combination, a refrigerated container, and the like, with the air of the conditioned space being controlled to a narrow temperature range around or adjacent to a predetermined set point temperature. Such systems conventionally utilize a chlorofluorocarbon (CFC) refrigerant in a mechanical refrigeration cycle. The mechanical refrigeration cycle requires a refrigerant compressor driven by a prime mover, which often includes a dedicated internal combustion engine, such as a diesel engine. Because of the suspected depleting effect of CFC's on stratospheric ozone (O.sub.3), and the detrimental effects of diesel emissions, practical alternatives to the use of CFC's are being sought.
The use of a cryogen, ie., a gas which has been compressed to a very cold liquid state, such as carbon dioxide (CO.sub.2) and nitrogen (N.sub.2), in a transport refrigeration system is particularly attractive because, in addition to eliminating the need for a CFC, it also eliminates the need to transport a compressor and associated prime mover.
It would be desirable, and it is an object of the present invention, to provide new and improved cryogenic based air conditioning and refrigeration systems, suitable for both transport and stationary applications, which more effectively and efficiently utilize the cryogen, for improved temperature control of a conditioned space, as well as for lower cost operation. The term "refrigeration system" will be hereinafter used to generically cover both air conditioning and refrigeration applications, transport and stationary.
Cryogens in a liquid state may be delivered to the refrigeration system at much colder temperatures than temperatures associated with fresh loads, such as lettuce, for example. Thus, it would be desirable, and it is another object of the invention, to provide a refrigeration system utilizing a cryogen which effectively conditions a space containing a fresh load, while minimizing the chance for spot freezing, without compromising the ability of the system to adequately condition a frozen load.